The present invention relates to hydraulic drive systems of the type including a pump-motor unit which operates as a pump during a portion of the vehicle operating cycle, and as a motor during another portion of the vehicle operating cycle. Even more particularly, the present invention relates to an improved control circuit for controlling the drive system, and a filter sub-system for use in such a hydraulic drive system.
Although the control circuit and the filter sub-system of the present invention may be utilized in hydraulic drive systems of various types, including such drive systems which effectively serve as the primary vehicle transmission during at least most of the vehicle operating cycle, the present invention is especially advantageous when used on a hydraulic drive system which comprises part of a vehicle hydraulic regenerative braking system, and will be described in connection therewith.
In a vehicle hydraulic drive system having regenerative braking capability, and assuming, by way of example only, that the vehicle is of the rear wheel drive type, the primary drive torque is transmitted from the engine through the conventional mechanical transmission, and then by means of a conventional drive line to the rear drive wheels. During braking (i.e., during the braking portion of a “deceleration-acceleration” cycle,) the kinetic energy of the moving vehicle is converted by the hydrostatic pump-motor unit, which is commanded to operate in its pumping mode, and the pump-motor unit charges a high pressure accumulator. When the vehicle is subsequently accelerated, the hydrostatic pump-motor unit is commanded to operate in its motoring mode, and the high pressure stored in the high pressure accumulator is communicated to the pump-motor unit. The resulting output torque of the pump-motor unit, now operating as a motor, is transmitted to the vehicle drive line.
It will be understood by those skilled in the art that there are several reasons why the present invention is especially suited for use in a drive system of the type described above, and which has regenerative braking capability. First, such a system typically includes not only the high pressure accumulator referred to, but also a low pressure accumulator. However, the presence of these two accumulators in the drive system complicates certain aspects of the configuration and the control of the drive system. Secondly, the presence of a pump-motor unit, which operates in a pumping mode for part of the vehicle cycle, and in a motoring mode for part of the vehicle cycle, introduces certain additional requirements and complications into the drive system and the controls therefor.
One of the complications which has been observed in a hydraulic drive system of the type to which the present invention relates, and which is used to accomplish regenerative braking, is the necessity to ensure proper filtration of the oil in what is essentially a “closed-loop” hydraulic system. In a conventional closed-loop hydrostatic transmission, or HST (i.e., a pump and motor combination), the pump almost always serves as a pump, and the motor almost always serves as a motor, during the normal propel operating cycle. In such a closed-loop HST system, it is conventional for some portion of the case drain fluid to be directed through a parallel circuit including elements such as a heat exchanger and a filter, after which that fluid is typically returned to the closed-loop circuit by means of a charge pump.
In the hydraulic drive system of the present invention, instead of a separate pump unit and motor unit, there is the above-described pump-motor unit. In view of the dual mode capability of the pump-motor unit of the type used in the hydraulic drive system of the present invention, it is not feasible simply to utilize the type of “parallel-path” filter circuit of the type typically utilized in closed loop HST systems, and described previously. In addition, whereas the “direction” of fluid flow in a typical closed-loop HST system remains the same throughout its operating cycles, in a hydraulic drive system of the type to which the present invention relates, many portions of the overall hydraulic system “see” fluid flow in one direction during one operating mode (e.g., deceleration) and “see” fluid flow in the opposite direction during the other mode (e.g., acceleration). As is well known to those skilled in the hydraulic circuit art, it is not feasible to utilize a conventional filter element in a circuit which experiences reversal of flow as part of its normal operation.
By way of example only, in a hydraulic drive system of the type to which the present invention relates, it is not advisable to locate a filter circuit or filter element in series flow relationship with the inlet of the pump-motor unit. When the pump-motor unit is operating in the pumping mode, the presence of a filter element in series with the pump inlet restricts pump inlet flow (especially after the filter element has collected a substantial amount of contaminant particles), thus resulting in cavitation of the unit (in the pumping mode) and excessive, undesirable noise emanating from the overall drive system. At the same time, it is not advisable to locate a filter element in series with the outlet of the unit (when it is operating in the motoring mode) because one result will be an increase in the total pressure drop across the unit, thus reducing the overall efficiency of the drive system. Another undesirable result would be that, as the filter element collects contamination particles, the pressure drop across the unit would vary, and therefore, the total system performance would also vary. If the filter element is located in series with the outlet of the unit (in the motoring mode) the filter element could rupture, and catastrophically contaminate the entire system. Moreover, because of the large flow rates involved, the filter element would have to be larger than is considered desirable, especially for mobile applications.